During adrenocortical cell differentiation changes occur in the smooth endoplasmic reticulum (SER), mitochondira and peroxisomes (Black, '72; Black and Bogart, '73) which can be correlated with the acquisition of steroidogenic capability. The SER proliferates and assumes complex configuration, mitochondira acquire characteristic shape and cristae morphology, and small perosisomes connected to the SER increase in number. SER and mitochondria possess most of the enzymes involved in steroids must shuttle between mitochondria and SER as they travese the biosynthetic pathway. The organization of enzymes on the membranes of these organelles and the intermembrane relationships which must determine the flow of steroids have not been studied in detail. Perosisomes are interconnected with the SER but their role in steroid metabolism in unclear. The major purposes of this study are five fold: 1) Determine by freeze-fracture and biochemical methods if there are sites of functional specilization in the SER. 2) Establish the route followed by steroids shuttling between SER and mitochondira. 3) Locate enzyme complexes in mitochondrial membranes. 4) Establish the function of the relationship between peroxisomes and SER. 5) Develop an in vitro system utilizing adrenocortical cells which will allow correlation to be made under controlled conditions between structural modifications and functional changes. BIBLIOGRAPHIC REFERENCES: Black, V. 1975 Effects of dexamethasone and ACTH on guinea pig adrenocortical cells. J. Cell Biol. 67: 32a. Robbins, E. and V. Black 1976 Plasma membrane organization in guinea pig adrenocortical cells from the zona fasiculata and zona reticularis. Anat. Rec. 184:515.